Strong and Weak Acids and Bases Green & Damjii – Chapter 8 – Section 3 Chang - Chapter 15 Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

An Arrhenius acid is a substance that produces H + (H 3 O + ) in water 4.3 There are multiple definitions for the terms ‘acid’ and ‘base’. You will need to be familiar with three (3) of them: Arrhenius acids and bases Bronsted-Lowry acids and bases Lewis acids and bases Today’s notes will focus on the Arrhenius definition!

Strong Electrolyte – 100% dissociation [like a soluble salt…or strong acids… or strong bases…] NaCl (s) Na + (aq) + Cl - (aq) H2OH2O Weak Electrolyte – not completely dissociated [like an insoluble salt… or weak acids… or weak bases] AgCl (s) Ag + (aq) + Cl - (aq) 15.4 Electrolyte – substance whose aqueous solution can conduct electricity – because of the presence of mobile ions H2OH2O

Strong Acids are strong electrolytes HCl (aq) + H 2 O (l) H 3 O + (aq) + Cl - (aq) HNO 3 (aq) + H 2 O (l) H 3 O + (aq) + NO 3 - (aq) HClO 4 (aq) + H 2 O (l) H 3 O + (aq) + ClO 4 - (aq) H 2 SO 4 (aq) + H 2 O (l) H 3 O + (aq) + HSO 4 - (aq) 15.4 HCl (aq) H + (aq) + Cl - (aq) H2OH2O H 3 O + is the hydronium ion General Equation for dissociation of a strong acid: HA (aq) + H 2 O (l) H 3 O + (aq) + A - (aq) or HA (aq) H + (aq) + A - (aq) H2OH2O

15.4 HF (aq) + H 2 O (l) H 3 O + (aq) + F - (aq) Weak Acids are weak electrolytes HNO 2 (aq) + H 2 O (l) H 3 O + (aq) + NO 2 - (aq) HSO 4 - (aq) + H 2 O (l) H 3 O + (aq) + SO 4 2- (aq) H 2 O (l) + H 2 O (l) H 3 O + (aq) + OH - (aq) General equation for the dissociation of a weak acid: HA (aq) + H 2 O (l) H 3 O + (aq) + A - (aq) or H2OH2O HA (aq)H + (aq) + A - (aq)

Strong AcidWeak Acid 15.4

percent ionization = Ionized acid concentration at equilibrium Initial concentration of acid x 100% For a monoprotic acid HA Percent ionization = [H + ] [HA] 0 x 100% [HA] 0 = initial concentration 15.5

Molecular Structure and Acid Strength H X H + + X - The stronger the bond The weaker the acid HF << HCl < HBr < HI 15.9

Molecular Structure and Acid Strength 15.9 NOTE: HF is a weak acid… H-F bond is very polar… which might make you think it should be easy to break… but H-F bond is VERY strong HF molecule can strongly hydrogen bond to water… (unlike the other hydrogen halides) stabilizing the undissociated molecule.

Ka – A Measure of Acid Strength Ka is the equilibrium constant for the dissociation of an acid – aka the acid dissociation constant a large Ka (> 1) means the products (dissociated ions) are favored – indicating a strong acid a small Ka (< 1) means the reactants are favored – indicating a weak acid

Ka – A Measure of Acid Strength NAME of ACIDFORMULAKa boric acidH 3 BO x 10 −10 hydrogen cyanideHCN5.9 x 10 −10 carbonicH2CO3H2CO3 4.3 x 10 −7 hydrogen sulfideH2SH2S1.3 x 10 −7 acetic acid CH 3 COOH 1.8 x 10 −5 formic acidHCOOH1.8 x 10 −4 hydrofluoric acid HF 6.9 x 10 −4 citric acidH3C6H5O7H3C6H5O7 7.4 x 10 −4 phosphoric acidH 3 PO x 10 −3 trichloroacetic acidCCl 3 COOH3.0 x 10 −1

Molecular Structure and Acid Strength Z O HZ O-O- + H + -- ++ In general, the acid will be stronger if the O-H bond is more polar and easier to break. This happens if: (1)Z (the non-metal) is more (very) electronegative The electronegativity of some common elements = N(3.0) Cl(3.0) S(2.5) C(2.5) P (2.1) So… …H 2 SO 4 is a stronger acid than H 3 PO 4 …HNO 3 is a stronger acid than H 2 CO

Molecular Structure and Acid Strength Here is another example, using the same rule… Oxoacids having different central atoms (Z) that are from the same group and that have the same oxidation number. Acid strength increases with increasing electronegativity of Z H O Cl O O H O Br O O Cl is more electronegative than Br, therefore HClO 3 is stronger than HBrO 3 HClO 3 > HBrO

Molecular Structure and Acid Strength Z O HZ O-O- + H + -- ++ In general, the O-H bond will be easier to break if: (2) Z (the non-metal) is in a high oxidation state – as indicated by an increase in the number of attached groups HClO 4 > HClO 3 > HClO 2 > HClO perchloric acid is stronger than chloric acid which is stronger than chlorous acid which is stronger than hypochlorous acid.

Molecular Structure and Acid Strength Which one of the acids in each of the following groups would be strongest? (a)H 2 SO 3 H 2 SO 4 sulfurous acidsulfuric acid (b)HNO 2 HNO 3 nitrous acidnitric acid (c)H 3 PO 3 H 3 PO 4 phosphorous acidphosphoric acid 15.9

What pattern do you notice between the formula and the name of these oxyacids? Using this pattern, what would be the formula for sulfurous acid?

Strong Acids hydrochloric acidHCl sulfuric acidH 2 SO 4 nitric acidHNO 3 hydrobromic acidHBr hydroiodic acidHI chloric acidHClO 3 perchloric acidHClO 4 Weak Acids ethanoic (or acetic) acidCH 3 COOH waterH2OH2O hydrofluoric acidHF phosphoric acidH 3 PO 4 formic acidHCOOH hydrocyanic acid hydrogen cyanide HCN hydrogen sulfideH2SH2S trichloroacetic acidCCl 3 COOH

How does acid strength relate to properties of acids? pH ? strong acids have lower pH than weak acids electrical conductivity? strong acids have greater electrical conductivity than weak acids reaction with active metals? carbonates? bases? strong acids react more vigorously than weak acids

An Arrhenius base is a substance that produces OH - in water 4.3

Strong Bases are strong electrolytes NaOH (s) Na + (aq) + OH - (aq) H2OH2O KOH (s) K + (aq) + OH - (aq) H2OH2O Ba(OH) 2 (s) Ba 2+ (aq) + 2 OH - (aq) H2OH2O 15.4

F – (aq) + H 2 O (l) OH – (aq) + HF (aq) Weak Bases are weak electrolytes NO 2 - (aq) + H 2 O (l) OH – (aq) + HNO 2 (aq) NH 3 (aq) + H 2 O (l) OH – (aq) + NH 4 + (aq) CH 3 CH 2 NH 2 (aq) + H 2 O (l) OH – (aq) + CH 3 CH 2 NH 3 + (aq)

Strong Bases lithium hydroxideLiOH sodium hydroxideNaOH potassium hydroxideKOH rubidium hydroxideRbOH cesium hydroxideCsOH calcium hydroxideCa(OH) 2 strontium hydroxideSr(OH) 2 barium hydroxideBa(OH) 2 Weak Bases ammoniaNH 3 waterH2OH2O ethyl amineCH 3 CH 2 NH 2 ammonium hydroxideNH 4 OH trimethylammonia(CH 3 ) 3 N pyridineC5H5NC5H5N

How does base strength relate to properties of bases? pH ? strong bases have ________ pH than weak bases electrical conductivity? strong bases have ________ electrical conductivity than weak bases reaction with acids? strong bases react ________ with acids than weak bases

NOTE: Do NOT confuse the strength of an acid or base with its concentration ! Circle the ‘strongest’ species in each pair below: (1) 6.0 M HCl vs 3.0 M HCl (2) 2.0 M HCl vs 2.0 M CH 3 COOH (3)0.3 M HCl vs 3.0 M CH 3 COOH (4) 0.3 M CH 3 COOH vs 3.0 M CH 3 COOH Underline the ‘least concentrated’ solution in each pair above.

Read Section PP Strong and Weak Acids & Bases Do Ex 8.3 on p. 214 # 1-5 (all) HOMEWORK: Due_______________